The proto-oncogene, c-fms, has been described in both mature cat macrophages and undifferentiated mouse and human tumors, and normal human bone marrow cells. The c-fms proto-oncogene is expressed during human monocyte differentiation and the c-fms product has been related to the receptor for mononuclear phagocyte growth factor (CSF-1). The purpose of this investigation is: (1) to screen for the presence of the c-fms proto-oncogene in the in vivo serially transplanted F344/N mononuclear leukemic cell lines and, (2) to initiate studies on the isolation and expression of the gene product. This will provide insight on the origin of the tumor and aid in providing diagnostic criteria for the tumor. In addition, this tumor line may provide a valuable source of genetic material for investigations into the molecular biology of the c-fms protooncogene and its potential effect on MNCL expression. Preliminary investigation and tentative conclusions indicate the possible presence of c-fms oncogene isolated from in vivo transplanted MNCL tumor lines.